US President Barack Obama has said he has not yet decided on a plan for retaliatory action against Syria.

But he said he had concluded Syrian government forces were behind a recent chemical weapons attack near Damascus.

Speaking on US television, Mr Obama said the use of chemical weapons affected US national interests and that sending a "shot across the bows" could have a positive impact on Syria's war.

His comments follow a day of behind-the-scenes wrangling at the UN.

The UK had been pushing for permanent members of the UN Security Council to adopt a resolution which would have authorised measures to protect civilians in Syria.

But Syrian ally Russia refused to agree to the resolution and the meeting produced no end to the diplomatic stalemate which has long characterised the UN position on Syria.

The US State Department criticised "Russian intransigence" and said it could not allow diplomatic paralysis to serve as a shield for the Syrian leadership.

Russia is sending an anti-submarine ship and a missile cruiser to the eastern Mediterranean.

The ships are being sent to strengthen the navy's presence in the area because of the "well-known situation" there, the Russian news agency Interfax has said.

But another news agency, RIA Novosti, quotes a senior naval command spokesman as saying that this is just a planned rotation, unconnected with Syria.

'Strong signal'

Critics have questioned what purpose a limited strike on Syria could serve, but Mr Obama told the Public Broadcasting Service (PBS) it would send the government of Bashar al-Assad "a pretty strong signal that it better not [use chemical weapons] again".

The US has yet to produce the intelligence it says shows Mr Assad's government is guilty of using chemical weapons, and UN weapons inspectors are still investigating inside Syria.

The team has just begun a third day of on-site investigations, and UN Secretary General Ban Ki-moon has appealed for it to be "given time to do its job". He said the inspectors would finish their investigations and be out of the area by Saturday morning.

Syria denies using chemical weapons and blames opposition fighters for the attack near Damascus on 21 August, which reportedly killed hundreds of people.

In a sign of growing fears about an impending attack among Syrians, the Associated Press quoted Lebanese officials as saying at least 6,000 Syrians crossed into Lebanon in a 24-hour period through the main Masnaa crossing - compared to a normal daily tally of between 500 and 1,000 refugees.

"Isn't it enough, all the violence and fighting that we already have in the country, now America wants to bomb us, too?" one 45-year-old woman, entering Lebanon with her five children, told AP.

In Damascus senior military commanders are reportedly staying away from buildings thought likely to be targeted. You "could hear a pin drop" at one of them, a local resident said.

'Consequences'

President Obama told PBS that the US had "not yet made a decision, but the international norm against the use of chemical weapons needs to be kept in place, and hardly anyone disputes that chemical weapons were used in a large scale in Syria against civilian populations".

“Start Quote

"We've looked at all the evidence, and we don't believe the opposition possessed chemical weapons of that sort," he said.

He added he had concluded that the Syrian government carried out the chemical weapons attack.

"There need to be international consequences, so we are consulting with our allies," he said.

There was "a prospect that chemical weapons could be directed at us - and we want to make sure that doesn't happen".

The BBC's David Willis in Washington says Mr Obama looked cautious and spoke in a measured way, and he was clearly concerned about getting Congress on board as well as the American public.

Opinion polls until now have shown very little interest among the US public in getting involved in the Syrian conflict.

In an open letter to the president, US House of Representatives Speaker John Boehner demanded he explain "the intended effect of military strikes", and how he would prevent the intervention escalating, if he wanted to win public and congressional backing for action.

More than 110 members of Congress have signed a letter formally requesting that Mr Obama seek congressional approval for any action in Syria.

US officials are expected to give senior members of Congress a classified briefing on the evidence that the Syrian government carried out the alleged chemical attack on Thursday.

Models for possible intervention

Iraq 1991: US-led global military coalition, anchored in international law; explicit mandate from UN Security Council to evict Iraqi forces from Kuwait

Balkans 1990s: US arms supplied to anti-Serb resistance in Croatia and Bosnia in defiance of UN-mandated embargo; later US-led air campaign against Serb paramilitaries. In 1999, US jets provided bulk of 38,000 Nato sorties against Serbia to prevent massacres in Kosovo - legally controversial with UN Security Council resolutions linked to "enforcement measures"

Somalia 1992-93: UN Security Council authorised creation of international force with aim of facilitating humanitarian supplies as Somali state failed. Gradual US military involvement without clear objective culminated in Black Hawk Down disaster in 1993. US troops pulled out

Libya 2011: France and UK sought UN Security Council authorisation for humanitarian operation in Benghazi in 2011. Russia and China abstained but did not veto resolution. Air offensive continued until fall of Gaddafi

The US has said it will not take action alone - but one of its primary allies, the UK, has agreed to wait until UN inspectors report back before taking a parliamentary vote on potential action.

Russia rejected a UK push to try to agree a resolution on Syria among permanent UN Security Council members on Wednesday, with Foreign Minister Sergei Lavrov saying the UN could not consider any draft resolution or proposed action in Syria before the UN weapons inspectors reported back.

The use of force without a sanction of the UN Security Council would be a "crude violation" of international law and "lead to the long-term destabilisation of the situation in the country and the region", Mr Lavrov has said.

UN 'moment'

The UK, US and France are continuing their discussions following the meeting of the five permanent members.

The UK will want to be seen to be exhausting every diplomatic avenue, says the BBC's Nick Bryant at the UN headquarters in New York.

For the UK, there needs to be a UN "moment" - despite the fact that UN action will likely again be blocked by Russia or China.

But even without UN backing the US and its allies have been clear that they see the military option still open to them, our correspondent says.

"This is the first use of chemical warfare in the 21st Century," said UK Foreign Secretary William Hague. "It has to be unacceptable... or we will confront even bigger war crimes in the future."

More than 100,000 people are estimated to have died since the conflict erupted in Syria in March 2011, and the conflict has produced at least 1.7 million refugees.

Forces which could be used against Syria:

Four US destroyers - USS Gravely, USS Ramage, USS Barry and USS Mahan - are in the eastern Mediterranean, equipped with cruise missiles

Cruise missiles could also be launched from submarines, including a British Trafalgar class boat. HMS Tireless was reportedly sighted in Gibraltar at the weekend

Airbases at Incirlik and Izmir in Turkey, and in Jordan, could be used to carry out strikes

Two aircraft carriers - USS Nimitz and USS Harry S Truman are in the wider region

The Royal Navy's response force task group- which includes helicopter carrier HMS Illustrious and frigates HMS Montrose and HMS Westminster - is in the region on a previously-scheduled deployment

RAF Akrotiri airbase in Cyprus could also be used

French aircraft carrier Charles de Gaulle is currently in Toulon in the western Mediterranean

French Raffale and Mirage aircraft can also operate from Al-Dhahra airbase in the UAE.

This page is best viewed in an up-to-date web browser with style sheets (CSS) enabled. While you will be able to view the content of this page in your current browser, you will not be able to get the full visual experience. Please consider upgrading your browser software or enabling style sheets (CSS) if you are able to do so.